The filaments of fluorescent lamps are covered with emission mix to facilitate passage of electrons through the gas for production of light. Over time, the emission mix is sputtered off of the filaments in normal operation, but a larger amount is sputtered off when the lamp is ignited with cold cathodes. When the emission mix becomes depleted, the lamp nears end-of-life (“EOL”). When the filament emission mix becomes depleted, a higher voltage is required for the cathodes to emit electrons. The other filament in the lamp may not have an equally depleted emission mix, therefore, electrons from the good cathode will bombard the depleted filament with electrons, but the depleted filament will require a higher voltage to force the electrons back to the good filament. This higher voltage results in an increase in temperature which may overheat the lamp and in some cases crack the glass if the lamp is not replaced.
Program-start ballast systems provide a longer life to fluorescent lamps by pre-heating the lamp filaments on startup before igniting the lamps, thereby mitigating emission mix depletion. Other conventional solutions to this problem involve detecting when a lamp is rectifying, which indicates a depletion of emission mix on a cathode, and turning off the ballast inverter to remove power from the lamps and thus prevent the lamp from overheating. This approach, however, also extinguishes all the non-rectifying lamps in parallel ballast configurations, which is undesirable since a user does not know which lamp has reached the EOL state. One solution increases the number of inverters in the ballast, for instance, where half of the lamps are powered from one inverter and the other half are powered from another inverter. This approach extinguishes half of the lamps connected to the ballast when a lamp is in rectification, leaving the other inverter powered to provide lighting while the EOL lamp is identified and replaced, but adds cost and complexity for every inverter added. Thus, there is a continuing need for cost-effective ballast with improved end-of-life protection.